Re: My Git Dev Blog - Week 9

On 19-Jul-2021, at 08:28, Kaartic Sivaraam [off-list ref] wrote:

Hi Atharva and all,

On 19/07/21 12:49 am, Kaartic Sivaraam wrote:

quoted

On 18/07/21 5:29 pm, Atharva Raykar wrote:

quoted

Passing the superprefix explicitly

My gut instinct tells me we could get away without having to go this way but
I haven't yet been able to figure out how. How 'prefix', 'super-prefix' and
'recursive-prefix' is still puzzling me. In case anyone has knowledge about
this please chime in and enlighten us. It would be very helpful.

I just noticed I was short and unhelpful here. To expand on what information would be
helpful on this respect, here's a snippet of a private e-mail I sent which I've
tweaked a little to make it relevant for the list audience:

-- 8< --

quoted

quoted

...

I took a look at this. I'm not able to find the exact issue but my gut instinct tells me
it has something to do with how prefix, super-prefix and recursive-prefix
are handled before and after the conversion. To be more specific, I'm having doubts
about whether the following code snippet[4] in shell ...

   prefix=$(git submodule--helper relative-path "$prefix$sm_path/" "$wt_prefix")
   wt_prefix=
   sanitize_submodule_env
   cd "$sm_path" &&
   eval cmd_update

... has been properly converted to C. Particularly, the fact that recursive calls to
'cmd_update' is made in the existing shell implementation in case '--recusive' is
passed needs to be observed well. This would subsequently result in multiple calls to
'cmd_init' with different 'prefix' and 'wt_prefix' values. So, that needs to be
observed well too.

The whole deal with prefixes confused me initially a lot as well, so I'll try to
explain what's going on there first, and then show why I had trouble with it.

The terminology of 'recursive-prefix' was something I borrowed from the update-clone
helper[5]. Just to give you a summary of which shell variable maps to which C
variable, along with my understanding of what they represent:

| shell      | C                | description                               |
|------------+------------------+-------------------------------------------|
| $prefix    | recursive_prefix | this represents the superproject prefix,  |
|            |                  | ie, the path of the superproject relative |
|            |                  | to the current subproject.                |
|            |                  |                                           |
| $wt_prefix | prefix           | this is the path into the working tree,   |
|            |                  | ie, the path of the directory from the    |
|            |                  | root of the git directory's worktree      |

The display paths that are generated for the error messages require one of these to
be non-NULL, but not both[6].

I feel confident the values of the various prefixes are handled properly during the
recursion because I setup a new worktree on based master as my "control", and a
combination of running with GIT_TRACE2 and printing the values showed me that both
of them handle the prefixes identically.

The issue with the subprocess approach[7] seems to be not with the prefixes, but
rather with the configuration not being properly read when it is set in the same
command invocation.

[5] https://github.com/git/git/blob/75ae10bc75336db031ee58d13c5037b929235912/git-submodule.sh#L536-L550
[6] https://github.com/git/git/blob/75ae10bc75336db031ee58d13c5037b929235912/builtin/submodule--helper.c#L258-L260
[7] https://github.com/tfidfwastaken/git/commits/submodule-helper-update-1a

I have not been able to get a complete picture of this myself. I'll try to take better
look later and see if I could get anything.

Note that this could also be me just not being able to get a proper picture of it.
Just wanted to share it in case others are able to get any ideas or able to think of a
possible cause.

[4]: https://github.com/git/git/blob/abb21c7263616f01c5e950861a29279ab21cb02f/git-submodule.sh#L654-L658
-- >8 --

I hope that gives a better idea about what kind of information would be helpful.
Kindly let me know if it doesn't.

Atharva,

In the meanwhile, I think you could continue polishing your attempted solution
and preparing to send it to the list. I'll try to dig more on my gut instinct
and let you know if I get anything. If you have some free time left after
polishing your series, you could try digging into it too :)

Sure. Thanks for the time and effort you are putting into this!

-- 
Sivaraam